Spin Evolution Of Neutron Stars Research Articles

Spin evolution of neutron stars in transient low-mass X-ray binaries

Spin evolution of neutron stars in transient low-mass X-ray binaries

Evolutionary Origin of Ultralong-period Radio Transients

Abstract Recently, two ultralong-period radio transients, GLEAM-X J162759.5-523504.3 (J1627) and GPM J1839-10 (J1839), were discovered with spin periods longer than 1000 s. The origin of these two ultralong-period radio transients is intriguing in understanding the spin evolution of neutron stars (NSs). In this work, we examine whether the interaction between strong magnetized NSs and fallback disks can spin NSs down to the observed ultralong period. Our simulations found that the magnetar + fallback disk model can account for the observed period, period derivative, and X-ray luminosity of J1627 in the quasi-spin-equilibrium stage. To evolve to the current state of J1627, the initial mass-accretion rate of the fallback disk and the magnetic field of the NS are in the range of (1.1–30) × 1024 g s−1 and (2–5) × 1014 G, respectively. In the active lifetime of the fallback disk, it is impossible for J1839 to achieve the observed upper limit of the period derivative. Therefore, we propose that J1839 may be in the second ejector phase after the fallback disk becomes inactive. Those NSs with a magnetic field of (2–6) × 1014 G and a fallback disk with an initial mass-accretion rate of ∼1024–1026 g s−1 are possible progenitors of J1839.

Are There Magnetars in High-mass X-Ray Binaries?* *Supported by the National Natural Science Foundation of China.

Magnetars form a special population of neutron stars with strong magnetic fields and long spin periods. About 30 magnetars and magnetar candidates known currently are probably isolated, but the possibility that magnetars are in binaries has not been excluded. In this work, we perform spin evolution of neutron stars with different magnetic fields in wind-fed high-mass X-ray binaries and compare the spin period distribution with observations, aiming to find magnetars in binaries. Our simulation shows that some of the neutron stars, which have long spin periods or are in widely-separated systems, need strong magnetic fields to explain their spin evolution. This implies that there are probably magnetars in high-mass X-ray binaries. Moreover, this can further provide a theoretical basis for some unclear astronomical phenomena, such as the possible origin of periodic fast radio bursts from magnetars in binary systems.

Spin evolution of neutron stars in wind-fed high-mass X-ray binaries

Abstract The observed X-ray pulse period of OB-type high-mass X-ray binary (HMXB) pulsars is typically longer than 100 seconds. It is considered that the interaction between the strong magnetic field of a neutron star and the wind matter could cause such a long pulse period. In this study, we follow the spin evolution of neutron stars, taking into account the interaction between the magnetic field and wind matter. In this line, as new challenges, we solve the evolution of the magnetic field of the neutron star at the same time, and additionally we focus on the effects of the wind properties of the donor. As a result, evolutionary tracks were obtained in which the neutron star spends some duration in the ejector phase after birth, then rapidly spins down, becomes quasi-equilibrium, and gradually spins up. Such evolution is similar to previous studies, but we found that its dominant physics depends on the velocity of the donor wind. When the wind velocity is fast, the spin-down occurs due to magnetic inhibition, while the classical propeller effect and settling accretion shell causes rapid spin-down in the slow wind accretion. Since the wind velocity of the donor could depend on the irradiated X-ray luminosity, the spin evolution track of the neutron star in a wind-fed HMXB could be more complicated than considered.

The spin evolution of neutron stars with the superfluid core

We investigate the neutron stars spin evolution (breaking, inclination angle evolution and radiative precession), taking into account the superfluidity of the neutrons in the star core. The neutron star is treated as a two-component system consisting of a "charged" component (including the crust and the core protons, electrons and normal neutrons) and a core superfluid neutron component. The components are supposed to interact through the mutual friction force. We assume that the "charged" component rotates rigidly. The neutron superfluid velocity field is calculated directly from linearized hydrodynamical equations. It is shown that the superfluid core accelerates the evolution of inclinaton angle and makes all pulsars evolve to either orthogonal or coaxial state. However, rapid evolution seems to contradict the observation data. Obtained results together with the observations may allow to examine the superfluid models.

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We have investigated the relation between the orbital periods (Porb) and the spin periods (Ps) of wind-fed X-ray pulsars in high-mass X-ray binaries (HMXBs), based on population synthesis calculations of the spin evolution of neutron stars during the pre-HMXB stage. We show that most neutron stars either have steady accretion or still reside in the radio pulsar phase when the donor star starts to evolve off the main sequence. In the former case, the spin period can be decelerated to ~102-103 s, depending on the value of Porb. We briefly discuss possible origins of the Ps-Porb correlation in Be/X-ray binaries and the existence of HMXBs with main-sequence donors. We also investigate the evolution of the inclination angle between the magnetic and spin axes of neutron stars in a massive binary, suggesting secular alignment of the magnetic and spin axes during their evolution.

Spin Evolution of Neutron Stars in OB/X-ray Binaries

We have investigated the relation between the orbital period Porb and the spin period Ps of neutron stars in OB/X-ray binaries. By simulating the time- development of the mass loss rate and radius expansion of a 20M donor star, we have calculated the detailed spin evolution of the neutron star before steady wind accretion occurs (that is, when the break spin period is reached), or when the OB star begins evolving o the main sequence or has filled its Roche lobe. Our results are compatible with the observations of OB/X-ray binaries. We find that in relatively narrow systems with orbital periods less than tens of days, neutron stars with initial magnetic field B0 stronger than about 3◊10 12 G can reach the break spin period to allow steady wind accretion in the main sequence time, whereas neutron stars with B0 < 3 ◊ 10 12 G and/or in wide systems would still be in one of the pulsar, rapid rotator or propeller phases when the companion evolves o the main sequence or fills its Roche lobe. Our results may help understand the various characteristics of the observed OB/neutron star binaries along with their distributions in the Ps Porb diagram.

A Ferromagnetic Model of Neutron Stars and the Spin Evolution of Anomalous X-Ray Pulsars

The spin evolution of neutron stars possessing a ferromagnetic core is investigated by focusing on the electromagnetic coupling of the core and the crust. Differential rotations of the core and the crust are shown to produce a huge energy dissipation in the crust owing to induction currents, thereby enhancing the stellar luminosity by two to three orders of magnitude beyond the dipole luminosity. Identifying the pulse period of a pulsar with the spin period of the core, the evolution of the pulse period involves a stationary era that is similar to that expected when a magnetic field decays. This ferromagnetic core model (FCM) is applied to the anomalous X-ray pulsars 1E 1841-045 and 1E 2259+586 to resolve some problems concerning their ages and luminosities. The importance of the braking index is noted to discriminate the FCM from the field-decay model.

On the spin evolution of neutron stars in pre-low-mass X-ray binaries

We present a simple model for the evolutionary states of neutron stars in pre-low-mass X-ray binaries during the main sequence stage of donor stars. It is shown that for typical parameters some of the neutron stars in these systems cannot accrete matter from winds of their companions in a stable way. Accretors are found if neutron stars have magnetic fields about 1013 G and higher and/or in close systems with orbital periods of about a few days and shorter.

Spin-down in the Anomalous X-Ray Pulsars

The anomalous X-ray pulsars (AXPs) are currently explained in terms of either magnetars—neutron stars with superstrong magnetic field strengths (~1014-1015 G)—or isolated, accreting neutron stars with normal surface magnetic field strengths (~1012 G). For the latter, AXPs were recently suggested to be neutron stars accreting from a surrounding disk formed by the supernova fallback matter. We have critically examined the spin evolution of neutron stars in the accretion model and found that it is unable to account for the distributions of the spin period, age, and X-ray luminosity of AXPs. A more promising explanation for such objects is the magnetar model.

R-mode oscillations of rapidly rotating Newtonian stars: A new numerical scheme and its application to the spin evolution of neutron stars

We have developed a new numerical scheme to solve r-mode oscillations of rapidly rotating polytropic stars in Newtonian gravity. In this scheme, Euler perturbations of the density, three components of the velocity are treated as four unknown quantities together with the oscillation frequency. For the basic equations of oscillations, the compatibility equations are used instead of the linearized equations of motion. By using this scheme, we have solved the classical r-mode oscillations of rotational equilibrium sequences of polytropes with the polytropic indices $N=0.5,$ 1.0, and 1.5 for $m=2,3,$ and 4 modes. Here m is the rank of the spherical harmonics ${Y}_{l}^{m}.$ These results have been applied to investigate the evolution of uniformly rotating hot young neutron stars by considering the effect of gravitational radiation and viscosity. We have found that the maximum angular velocities of neutron stars are around 10--20 % of the Keplerian angular velocity irrespective of the softness of matter. This confirms the results obtained from the analysis of r-modes with the slow rotation approximation employed by many authors.

Magnetic and spin evolution of neutron stars in close binaries

The evolution of neutron stars in close binary systems with a low-mass companion is considered, assuming the magnetic field to be confined within the solid crust. We adopt the standard scenario for the evolution in a close binary system, in which the neutron star passes through four evolutionary phases (‘isolated pulsar’–‘propeller’– accretion from the wind of a companion – accretion resulting from Roche-lobe overflow). Calculations have been performed for a great variety of parameters characterizing the properties of both the neutron star and the low-mass companion. We find that neutron stars with more or less standard magnetic field and spin period that are processed in low-mass binaries can evolve to low-field rapidly rotating pulsars. Even if the main-sequence life of a companion is as long as 1010 yr, the neutron star can maintain a relatively strong magnetic field to the end of the accretion phase. The model that is considered can account well for the origin of millisecond pulsars.

Spin and magnetic evolution of accreting neutron stars in close binaries

The paper considers the influence of magnetic field decay on the spin evolution of neutron stars in the course of an accretion phase. The simplest model is assumed for the interaction of the accreted matter with the neutron star magnetosphere. We analyse a few possible decay laws of the magnetic field which can be representative of various theoretical scenarios. For the considered decay laws, the neutron star entering a low-mass binary system can generally spin up to millisecond periods if it accretes 0.01-0.1 Mo. Our calculations show that a power-law decay of the magnetic field with the power P < 2 is most suitable for the evolution of 'standard' neutron stars with B ~ 1011 _1013 G to millisecond pulsars with B ~ 108_109 G.

Spin evolution of neutron stars in binary systems

I review our understanding of the evolution of the spin periods of neutron stars in binary stellar systems, from their birth as fast, spin-powered pulsars, through their middle life as accretion-powered pulsars, upto their recycling or “rebirth” as spin-powered pulsars with relatively low magnetic fields and fast rotation. I discuss how the new-born neutron star is spun down by electromagnetic and “propeller” torques, until accretion of matter from the companion star begins, and the neutron star becomes an accretion-powered X-ray pulsar. Detailed observations of massive radio pulsar binaries like PSR 1259-63 will yield valuable information about this phase of initial spindown. I indicate how the spin of the neutron star then evolves under accretion torques during the subsequent phase as an accretion-powered pulsar. Finally, I describe how the neutron star is spun up to short periods again during the subsequent phase of recycling, with the accompanying reduction in the stellar magnetic field, the origins of which are still not completely understood.

Effect of radiation forces on disk accretion by weakly magnetic neutron stars

Radiation forces are shown to be more important than general relativistic corrections to Newtonian gravitational forces in determining the motion of particles accreting onto a slowly rotating neutron star if the luminosity of the star is greater than about 1 percent of the Eddington critical luminosity (ECL). This is so even if the radius of the star is less than the radius of the innermost stable orbit. In particular, radiation drag causes matter accreting from a disk to lose angular momentum and spiral inward. At luminosities greater than about 0.2 ECL, a substantial fraction of the accreting matter can transfer most of its angular momentum and gravitational binding energy to the radiation field before reaching the stellar surface. These results have important implications for the X-ray spectra, time variability, and spin evolution of neutron stars with very weak magnetic fields and the prospects for detecting general relativistic effects near such stars.